This invention relates to a thermoplastic elastomer composition. More particularly, this invention relates to a novel thermoplastic elastomer composition which is rich in flexibility and excellent in rubber elasticity over a broad temperature range, high-temperature creep characteristics, low-temperature impact resistance, mechanical strength and moldability, have good oil resistance, good light discoloration resistance and extremely excellent toning properties in spite of thermoplastic elastomer, and can be used as a material for various moldings.
Thermoplastic elastomers which are rubbery soft materials, which do not require curing step and which have the same moldability as thermoplastic resins, have recently been utilized in the fields of automobile parts, domestic appliance parts, wire-coating materials, medical parts, miscellaneous goods, footwear and the like.
Typical examples of the structure of a thermoplastic elastomer includes a type in which a hard segment and a soft segment are alternately present in a copolymer chain as disclosed in Japanese Patent Application Kokai No. 61-34,050 or the like. By varying the proportion of each segment, thermoplastic elastomers of various grades from flexible to rigid have been produced.
Moreover, there are different kinds of thermoplastic elastomers derived from inexpensive and easily available materials. That is to say, as disclosed in Japanese Patent Application Kokoku No. 53-21,021 or the like, a thermoplastic blend of a monoolefin copolymer rubber partially crosslinked with an organic peroxide and a polyolefin resin, and a partially crosslinked composition obtained by melt-kneading a monoolefin copolymer rubber and a polyolefin resin using an organic peroxide as a crosslinking coagent, correspond to the above different kinds of thermoplastic elastomers.
However, in the case of the former thermoplastic elastomer having such a structure that a hard segment and a soft segment are alternately present in the copolymer chain, it is necessary for the copolymer to contain a large amount of the soft segment for obtaining a flexible thermoplastic elastomer. Usually, the soft segment is low in tensile strength and inferior in heat resistance, flow properties and oil resistance, and hence, the flexible thermoplastic elastomer containing a large amount of such a soft segment is also low in tensile strength and inferior in heat resistance, flow properties and oil resistance, and therefore, cannot be applied to various uses over a broad range.
When thermoplastic elastomers of various flexibility grades are intended to be synthesized by a multistage synthesis process, it is necessary to separately synthesize the soft segment and the hard segment, so that the polymerization apparatus becomes very complicated and also it is very difficult to control the properties and proportion of each segment in the polymerization stage. Moreover, in some cases, many defective products are produced at the time of change of the grade. Furthermore, the recovery of the polymer produced is very difficult because a large amount of rubbery material is contained in the polymer.
In the case of the latter thermoplastic elastomer composition having such a structure that a partial crosslinking has been imparted to a monoolefin copolymer rubber which is one of the constituents, the oil resistance, the shape recovery at high temperatures and the like are insufficient because of the partial cross-linking, so that the thermoplastic elastomer cannot be applied in various uses over a broad range. Also, since an organic peroxide is used, the polymer chain is cut by a radical resulting from the organic peroxide simultaneously with the crosslinking and a reduction of mechanical strength is observed. Japanese Patent Application Kokoku No. 58-46,138 and others disclose a measure for overcoming the above disadvantages. This is such that a heat-reactive alkylphenol resin is used as the cross-linking agent to allow only the crosslinking of the monoolefin copolymer rubber to proceed preferentially. The thermoplastic elastomer obtained by this measure is completely crosslinked, and hence, is sufficient in oil resistance, shape-recovery at high temperatures and the like. However, since the alkylphenol resin is used, the light discoloration resistance is greatly inferior, and hence, the thermoplastic elastomer cannot be applied to uses requiring the freedom of coloration such as automobile parts, domestic appliance parts, wire-coatings and the like.
Moreover, U.S. Pat. No. 4,803,244 proposes using an organosiloxane compound in place of the alkylphenol resin as a cross-linking agent. According to this method, it is possible to allow only the crosslinking of the monoolefin copolymer rubber to proceed preferentially similarly to the crosslinking with an alkylphenol resin, and a material extremely excellent in oil resistance, shape recovery at high temperatures, light discoloration resistance and the like can be obtained, so that the resulting thermoplastic elastomer can be applied to uses such as automobile parts, domestic appliance parts, wire coatings and the like which require freedom of coloration. However, according to this method, no compatibilizing agent is used, and hence, the adhesion between the rubber and the resin is not sufficient, so that the impact resistance at low temperatures cannot be said to be sufficient. Therefore, such a thermoplastic elastomer cannot be applied to uses requiring impact resistance at low temperatures, for example, materials for air bag cover of an automobile. Also, in the case of a usual kneading method, the current situation is that since no compatibilizing agent is used the rubber dispersion is not sufficient and hence the appearance of the extrusion-molded article is remarkably inferior. Therefore, as disclosed in U.S. Pat No. 4,594,390, dynamic heat-treatment under a high shear of 2000/sec or higher has been proposed for solving the above problem. However, according to this method, the material is exposed to high shear though the residence time is short, and therefore, the material per se tends to decompose and deteriorate. Moreover, in order to achieve high shear, it is necessary not only to increase the revolution rate in the melt-kneading but also narrow the clearance of a kneader to about 1/3 to 1/5 of that of a usual kneader. Thus, said method is not practical in the aspect of productivity of a thermoplastic elastomer composition and durability of a kneader.
By the known techniques which have been proposed by now, the essential problem has not been solved, that is, the morphologic change that the rubber dispersed by an annealing treatment is agglomerated again is still caused, and as a result, the current situation is that the thermoplastic elastomer composition cannot be used in the automobile field or the like wherein the long term reliability of quality is required.